Low frequency amplifier



March 14, 1939.

R. F. voLz ET AL 2,150,567 LOW FREQUENCY AMPLIFIER Filed Jan. 4,. 1956 AAAIAAAA Patented Mar. 14, 1939 2,150,567 LOW FREQUENCY AMPLIFIER Rudolf Friedrich Volz and Abraham Jan Werner de Grout, Eindhoven, Netherlands, assignors to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Application January 4, 1936, Serial No. 57,548 In Germany April 15, 1935 4 Claims. (01. 179-471) This invention relates to amplifiers comprisvalve, whereby said valve may receive additional ing thermionic valves for low frequency oscillabias potential. Such a resistance may be a self tions such as those of audio frequency and has bias resistor provided for automatically obtaining for its object to provide an improved circuit ara negative bias for the amplifier. This resistance rangement such that the amplification of powerwould bring about an increase of the negative 5 ful signals is greater than that of feeble signals. bias in the case of an increase of the signal in- Such circuit arrangements are wel1 known and tensity because of the tendency to increase the are commonly referred to as volume expanders. p a e current of t e p fi W increased They are particularly applicable to talking maplate voltage, so that the action above describedchine amplifiers. As is well known, with records, would be partially neutralized. If in accordance the ratio of the amplitudes of the sound track with the invention, the cathode of the control should always be, in the case of powerful and tube or valve is connected to a proper point on feeble points, substantially smaller than correthis resistance, the grid bias may be made consponds to the actual volume or intensity ratio. stant or may be made to decrease with increas- The use of an amplifier of this type will thereing signal voltage.

fore ensure that reproduction approaches reality In order that the invention may be clearly unmore closely. In wireless receiving sets such amderstood and readily carried into effect, one cirplifiers can also often be used with advantage cui-t arrangement in accordance therewith will since the percentage variation of the modulation now be described more fully by way of example,

of the broadcasting waves is generally lower than with reference to the accompanying drawing. 29

the variation in intensity of the sound oscillations Referring to the drawing, corresponding thereto. Figure 1 is a wiring diagram of an amplifier It has previously been proposed to rectify part constructed and arranged in accordance with the of the output energy and lead it back to the grid invention, circuit of the first valve so that the amplification Figure 2 illustrates the application of the in- 2.5 ratio of this valve is automatically controlled in vention to a plurality of different frequency accordance with the signal amplitude. In the channels, and

well known arrangements the negative bias may Figure 3 illustrates certain details of an amreadily decrease to an excessive extent on the plifier similar to that of Figure l or 2.

Bil occurrence of a powerful signal sothat distortion Referring to Figure l, l designates the first ensues. This necessitates the use of devices valve of an, amplifier. The signals to be ampliwhich limit this effect. fied are received at the terminals 6. The an- According to the invention, the improved arode current is passed through a coupling rerangement comprises an amplification ratio consistance 4. The anode is in addition connected trol valve which does not serve for the amplificaacross a coupling condenser I3 to the diagram- 3: tion of the low frequency oscillations and which matically illustrated amplifier 3 which may comreceives its anode current through resistance inprise in addition one or more amplifying stages cluded in the anode circuit of one of the amplifier and which supplies its output energy through an valves. It comprises in addition, a rectifier that output transformer l2 to the load, for example, 4i rectifies part of the output energy and supplies a loud-speaker. A further part of the output 40 it to an impedance connected into the grid cirenergy is supplied through a condenser M to a cult of the first-mentioned valve, the arrangepotentiometer ll provided with a tap 16 which is ment being such that when the signal voltage connected through a condenser It to a rectifier increases the anode current of this valve deanode 8.

r a re y decreasing h potential p An amplification ratio control valve 2 is conthrough the common resistance above referred nected in parallel with the valve I, the anodes of to, whereby the mean a flde V lt e of the amthe valves and 2 being interconnected directly. Plifier valve increases, causing the amp ifica o The cathode of the valve 2 is connected to a point to increase. The mutual conductance then be- I on the resistance 5 which is included in the comes higher because of the decrease in the outcathode lead of the valve l. The resistance in 50 put or plate resistance of the valve. serted in the cathode circuit is preferably capable In addition, according to the invention, the of being varied by means of a contact I3. The cathode of the first mentioned or control valve resistance 5 serves for obtaining a negative bias may be connected to a point on a resistance infor the grid of the valve l. cluded in the cathode conductor of the amplifier The rectifier anode 8 is mounted in the valve 2 55 and the rectified current flows from the cathode of this valve across the resistance 9, one end of which is connected to the grid of the valve 2 and the other end of which is connected to the cathode of the valve 2. The arrangement comprises in addition a balancing device or filter means I5 which may also serve for introducing a given time constant dependent, as is well known, upon the capacity of the filter means and the amount of series resistance, indicated at I5a, which may be provided. In the present example, the filter is the usual series impedance, shunt capacitor type, of low pass filter interposed in the grid connection for valve 2 with the rectifier output resistor 9.

' The arrangement operates as follows:

Signals applied to the amplifier I through the input terminals are amplified thereby, and, after further amplification in the amplifier 3, are applied to the receiver through. the potentiometer connection as previously pointed out.

In case of a feeble or low amplitude signal applied to the terminals 6-45, the rectified compcnent thereof, derived from the resistor II through the rectifier 8 will result in only a feeble current passing through the rectifier output resistance 9 so that the grid of the valve 2 is caused to be but slightly negative. The current then flowing through the valve 2 is great or relatively high and the voltage at the anodes of the two valves is at a low value because of the potential drop set up 'by the current fiow through the common anode resistor 4. In addition, the voltage drop across the resistance 5 is high because the same anode current through resistor 4 also flows in resistor 5, and thus the negative bias of the grid of the valve I is also high. Under this condition the amplification of the valve I is low.

In the case of a. powerful signal the rectified component in the output resistor 9 ishigher and the grid of the valve 2 becomes more highly negative; Therefore the anode current falls, for example, down to or toward zero and the voltage at the anode of valve I increases because of the decreased potential drop in the resistor 9. The negative bias at the grid of the valve I decreases because of the decreased potential drop in the resistor 5 and decreased negative bias, and the amplification increases. In this manner, the contrast between weak and strong signals is consequently intensified. Instead of being arranged in the valve 2, the rectifier may be arranged independently. In either case it applies to the grid of the control valve I a bias potential which varies in response to change in the output current of the amplifier 3.

The contacts I and I6 are preferably slidable. The former permits of adjusting the variation of the negative bias of the grid of the valve I in the case of a variation of the signal intensity. The latter permits of adjusting the operating effect of the control valve 2 and thus the intensity of the contrast between the amplification ratios at different signal strengths. The latter contact may be coupled mechanically to the knob serving for the volume control which permits of ensuring that even in the case of adjustment to low strength of sound the contrast action is still maintained or is varied in accordance with a given law.

It is desirable to provide a filter I! which serves to filter the high frequencies out of the energy derived from the amplifier 3 for the control of valve 2. This has the advantage that the contrast or expander action is not interfered with by disturbances (in the case of wireless receipt) and needle noise (in the case of talking machine reproduction). It may be possible to filter out or to weaken at the same time very low frequencies so that the contrast action is not interfered with by any particularly low sounds in a given musical composition.

If, as is often the case, use is made of a low frequency amplifier arrangement subdivided into a plurality of parallel-connected amplifiers each of which serves to amplify a given frequency band, the signals are preferably severed subsequent to the amplification by a valve I. This is diagrammatically illustrated in Fig. 2 wherein V is the amplifying stage thatcomprises the valves I and 2, V1 to V4 are the subsequent amplifying stages. The voltage required for the adjustment of the grid bias of the valve 2 is derived from one or more of the amplifiers, preferably from those that do not contain the very high or the very low frequencies, consequently, for example, Va and V3.

Figure'3 shows the manner in which the voltages are rectified and can be supplied to the grid of the valve 2 The valve comprises two rectifying anodes 8 and 8 which are connected to the cathode through resistances 9 and 9'. The arrangement comprises in addition two filters I5 and I5 for balancing the rectified signal voltage. The grid is connected to the output points of these filters.

What we 'claim is:

1. In a volume expander amplifier, the combination of an amplifier valve having a grid circult and an anode output circuit, a control valve provided with an anode circuit connected in parallel with said amplifier anode circuit, said second anode circuit including a potential drop producing impedance incommon with said first anode circuit, a rectifier, means providing a rectifier circuit connected to and energized from said amplifier output circuit and having a rectifier output impedance, said grid circuit being connected to said rectifier circuit output impedance to receive a biasing potential therefrom, and a circuit including a variable impedance device through which current is supplied to said rectifier circuit and a grid bias potential device common to said parallel connected anode circuits and to said amplifier grid circuit.

2. In a volume expander amplifier, the combination of an amplifier valve having a grid circuit and an anode output circuit, a control valve provided with an anode circuit connected in parallel with said amplifier anode circuit, said second anode circuit including a potential drop producing impedance in common with said first anode circuit, a rectifier, means providing a rectifier circuit connected to and energized from said amplifier output circuit and having a rectifier output impedance, said grid circuit being connected to said rectifier circuit output impedance to receive a biasing potential therefrom, a circuit including a variable impedance device through which current is supplied to said rectifier circuit, an impedance device common to said amplifier grid and said parallel connected anode circuits, and means for adjusting a portion of said impedance included in said grid circuit.

3. The combination of an amplifier valve including an anode, a cathode and a control grid, a rectifier having an anode, means providing a variable connection from said rectifier to said amplifier valve anode, said connection including a filter and a control valve connected to and energized from said rectifier anode, said control said tubes, said last named resistor being included in circuit with the grid of the amplifier tube to apply biasing potential thereto, means for adjusting said last named potential, means for deriving an amplified signal from said amplifier 5 including a potentiometer device having a variable output connection, a rectifier device included in said connection, said rectifier having an output resistor, a control grid for said control tube connected with said output resistor, and a 0 time delay filter in said last named connection. RUDOLF FRIEDRICH VOLZ. ABRAHAM JAN WERNER DE GROOT. 

